قدمه على علم البيان؛ لكونه منه بمنزلة المفرد من المركب
He placed maʿani before bayan because, in one respect, their relation resembles that of the simple to the composite.
This is an analogy for pedagogical order, not a literal claim that the sciences are individual words and compounds. Maʿani establishes appropriateness to the occasion; bayan adds the study of different ways to convey a meaning while preserving that appropriateness.
Thus the structure and fitness of discourse precede detailed inquiry into figurative presentation.